1. Field of the Invention
The present invention relates to a method of and apparatus for controlling a fuel injection quantity for an internal combustion engine. More particularly, the present invention pertains to a method of and apparatus for controlling a fuel injection quantity for an internal combustion engine in which the fuel injection quantity is increased synchronously or asynchronously with the crank angle during acceleration.
2. Description of the Related Art
One type of fuel injection quantity control apparatus has heretofore been known in which a basic fuel injection quantity is determined on the basis of an intake-pipe pressure (absolute pressure) PM and a rotational speed of the engine NE, and fuel is injected every predetermined crank angle (i.e., in synchronism with the crank angle) on the basis of the basic fuel injection quantity which is corrected in accordance with, for example, the temperature of intake air and the temperature of water for cooling the engine. In this type of fuel injection quantity control apparatus, in order to improve the response of the engine at the time of acceleration, an amount .DELTA.PM of change in the intake-pipe pressure PM is detected, and fuel to be injected is increased synchronously with the crank angle by an amount which is proportional to the detected change amount .DELTA.PM (see Japanese Patent Laid-Open No. 35154/1985).
In the above-described fuel injection quantity control, since the intake-pipe pressure rises at a substantially constant rate during acceleration, the change amount .DELTA.PM of the intake-pipe pressure is substantially constant, so that a substantially constant amount of fuel is incrementally injected during acceleration. However, the intake-pipe pressure increases in proportion to the degree of opening of the throttle valve. More specifically, at the beginning of acceleration, the intake-pipe pressure is relatively small, whereas, at the end of acceleration, the intake-pipe pressure is relatively large. For this reason, although at the beginning of acceleration the amount of fuel which adheres to the inner wall of the intake manifold is relatively small, a relatively large amount of fuel adheres to the manifold inner wall at the end of acceleration. Accordingly, the conventional fuel injection quantity control in which a substantially constant amount of fuel is incrementally injected during acceleration involves the problem that, at the beginning of acceleration, the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber of the engine is richer than the level required by the engine, whereas, at the end of the acceleration, said air-fuel ratio becomes leaner than the required level. In consequence, although the air-fuel ratio is controlled to the required level of the engine in some regions during acceleration, it is impossible to control the air-fuel ratio to the required level throughout the acceleration, which means that the engine performance during acceleration is still unsatisfactory.
In order to improve the response of the engine at the time of acceleration, one type of internal combustion engine is adapted to execute asynchronous fuel injection in which fuel is injected asynchronously with the crank angle, in addition to the synchronous fuel injection in which the fuel is injected synchronously with the crank angle. The asynchronous fuel injection enables the amount of fuel supplied to the engine to be increased even when the intake pipe pressure rises rapidly during acceleration, so that the air-fuel ratio during acceleration can be made closer to the required level of the engine.
Examples of conventional methods for carrying out the above-described asynchronous fuel injection include a method wherein the output of a pressure sensor which is input to control means through a filter is sampled with a predetermined period, and when the difference between the presently sampled value and the previously sampled value exceeds a predetermined value, asynchronous fuel injection is executed (see Japanese Patent Laid-Open No. 90728/1984), and a method wherein the output of a pressure sensor which is input to control means through a filter is subjected to differentiation of second order with respect to time, and when the result of the differentiation exceeds a predetermined value, asynchronous fuel injection is executed (see Japanese Patent Laid-Open No. 39938/1984).
However, the filter which is used in these conventional methods has a time constant set at a relatively large value for the purpose of removing the pulsating component of the intake-pipe pressure (i.e., since the level of the pusating component reaches a maximum when the throttle valve is in a full-open position, it is necessary, in order to remove the pulsating component in all operating conditions, to set the time constant in conformity with circumstances at the time when the throttle valve is in a full-open position). For this reason, the rise of a signal which is input to the control means is delayed with respect to an actual change in the intake-pipe pressure, and this undesirably delays the timing at which asynchronous fuel injection should be started. Accordingly, asynchronous fuel injection is not executed at the very beginning of acceleration, and the performance of the engine at the beginning of acceleration is degraded. In addition, since asynchronous fuel injection is started at the time when the early stage of acceleration has passed and the intake-pipe pressure has already become relatively high, the amount of evaporated fuel decreases, and a relatively large amount of fuel adheres to the inner wall of the intake manifold, resulting, disadvantageously, in a reduction in amount of fuel supplied to the engine.
To overcome the above-described problems and improve the engine performance during acceleration by the fuel injection quantity control effected on the basis of a change amount .DELTA.PM and by eliminating the delay in starting of asynchronous fuel injection, it may be effective practice to input the pressure sensor output to the control means through a filter having a time constant set at a relatively small value (e.g., 3 to 5 msec) so as to remove only a minimum pulsating component of the intake-pipe pressure. In such case, however, since the pulsating component cannot completely be removed by the filter, the change amount .DELTA.PM may vary due to the remaining pulsating component, resulting, undesirably, in an excess increase in the amount of fuel injected although a steady-state running condition. If, in order to solve this problem, the threshold value employed to effect increment of fuel is raised, no increment of fuel can be effected at the time of, for example, slow acceleration. Further, when the temperature of the engine is relatively low, it is necessary to increase the amount of fuel injected in proportion to the rise in the intake-pipe pressure even in the latter part of acceleration during which the change amount .DELTA.PM in the intake-pipe pressure becomes small. However, in such case, since the change amount .DELTA.PM in the intake-pipe pressure is small, the required increase in the amount of fuel cannot be achieved. In order to carry out increment of fuel in proportion to the change amount .DELTA.PM even in the latter period of acceleration, it is necessary to effect a complicated control such as one in which the time constant of the filter is made variable.
With respect to the asynchronous fuel injection also, if a filter having a relatively small time constant is employed, the pulsating component cannot completely be removed by the filter, and this involves the problem that the difference in terms of sampled values or a value obtained by differentiation of second order may exceed a predetermined value not only at the time of acceleration but also at the time of a steady-state running condition wherein the pulsating componet is relatively large, resulting in undesirable execution of asynchronous fuel injection. Such problem can be solved by increasing the above-described predetermined value. However, if said value is increased, asynchronous fuel injection cannot be executed at the required timing during acceleration. Accordingly, the engine performance during acceleration is degraded, and the condition of exhaust emission becomes worse.